l2: coverity woe in l2_api.c

l2: coverity woe in l2_api.c

Coverify complains deref_ptr before null check.

    	deref_ptr: Directly dereferencing pointer reg.
1214              vl_reg = vl_api_client_index_to_registration (reg->client_index);
1215              ALWAYS_ASSERT (vl_reg != NULL);
1216

CID 216104 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking reg suggests that it may be null, but it
 has already been dereferenced on all paths leading to the check.
1217              if (reg && vl_api_can_send_msg (vl_reg))

I believe the check is for vl_reg instead of reg because vl_reg may be NULL
after the call vl_api_client_index_to_registration.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Ic4eb2284e65c48396f20d5024a4241c80c70c886